Existing strategies for detecting air in the line of an infusion device often involve the use of ultrasonic sensors that are physically located on opposite sides of a tubing segment. When fluid is present in the tube, propagation of the acoustic signal is efficient and produces a large electrical signal via the receiver circuit. On the other hand, the presence of air in the tube causes an acoustical open circuit which substantially attenuates the detected signal. In current practice, detection of air in the tubing segment is performed on the basis of a simple (static) air-fluid boundary or threshold that is applied to the sensor voltage signal. When the air sensor signal moves beyond the pre-defined air/fluid threshold, an alarm condition occurs and the IV infusion is paused.
Additionally, in current practice, there exist methods/algorithms that utilize the plunger force sensor readings to detect the presence of air in the plunger chamber. Several Hospira™ pumps involve the use of a cassette with a chamber that is compressed by an actuated plunger to pump fluid at a controlled rate from the drug container to the patient. The measured force during a pumping cycle is directly related to the type of fluid in the chamber. For instance, fluids are relatively incompressible and generate a higher and different force profile than air. Similarly, a combination of fluid and air in the chamber results in a hybrid force profile that is indicative of the mixture percentages.
Both methods described above rely on observations from a single sensor (i.e., air sensor or force sensor). Faulty sensor observations are the major drawback of such single-sensor based systems/algorithms. For instance, for air sensor based algorithms, a variety of situations (e.g., dancing micro air bubbles, stuck fluid droplet at the end-of-bag, etc.) exist which generate false alarms or mask the presence of air in front of the air-sensor leading to false negatives. Similarly, force sensor based algorithms can be fooled by variable distal/proximal pressure during delivery (e.g., kinked tubing due to patient movement). The measured force during a pumping cycle is affected by the pressure applied to both distal and proximal sides of the tubing. For instance, drop in a distal pressure will cause drop in the plunger force readings, which will be perceived as a transition from fluid to air in the chamber by the existing force algorithms and cause a false positive detection of air. Single-sensor based air-in-line detection systems may fail to detect an end-of-bag situation that can result in air in the line, or may incorrectly determine that the fluid in the line is air (i.e., causing nuisance alarms).
A system and method is needed to overcome one or more issues of one or more of the existing infusion systems or methods.